The present invention relates generally to printing plates, and more particularly, is directed to an apparatus and method for the calibration of laser ablateable printing plates.
Printing plates can be made of different materials. For example, in a flexographic printing machine, a laser ablateable printing plate can be used as the printing medium. In such case, the laser ablateable printing plate can be produced from a variety of polymeric and elastomeric materials.
An image can therefore be formed on the printing plate using a process of laser ablating. In such case, a laser cuts away or ablates material from the printing plate, and the remaining non-ablated or raised portions represent the image to be used on the printing plate in a printing operation. In this manner, the laser ablates excess material from the surface of the plate, leaving only those areas which are needed for printing. In order to control this laser ablating operation, the images to be ablated on the printing areas are digitally input to the laser.
However, variations exist in the ablatability of different plate materials, whereby one plate material will ablate differently from another plate material, even when the image from the laser is the same, and the laser performance remains consistent. Further, the speed of laser ablating is directly proportional to the available energy of the laser and the relief depth of ablating. In order to maximize laser ablating speed, it is desirable to set the laser energy to the correct level needed to ablate a particular relief depth.
Therefore, it is necessary to know how deep to laser ablate the material in the printing plate. For example, 80% laser energy will cut deeper for one material than another material. Therefore, it is necessary to provide a calibration for the particular materials in order to set the laser energy, that is, the laser power and speed/duration of laser power, during a laser ablating operation. For example, the longer that the laser is applied at a given power, the more energy that is supplied for ablating. Therefore, both laser power and speed of the laser (which corresponds to the duration of laser power) will change the amount of laser energy that is applied.
In order to accomplish this, it is necessary to make adjustments to the laser parameters for laser ablating different plate materials in order to obtain a consistent ablated image. This process is generally known as material calibration. Specifically, it is known in the prior art to provide a number of laser ablations on a plate of a known material, and then, the depth of each ablated or engraved area is manually measured. Thus, the existing method for calibration of a laser ablateable plate material includes the steps of ablating the material using a range of laser energies, from 0% laser energy to 100% laser energy, and then manually measuring the depth of the ablations. The laser energy is then adjusted to suit the target relief depth.
A number of variables can affect the performance and accuracy of the calibration, including, but not limited to:
a) the resolution of the digital image being ablated;
b) the ablatability of the plate material;
c) the number of steps of laser energy being used for ablating (i.e. from 0% to 100%);
d) small variations in laser energy during the calibration process; and
e) operator errors in manual measurement of ablated depths.
As a result, this is a cumbersome and burdensome process, and requires a relatively long period of time. Although it is generally accepted that a higher number of steps of laser energy that are used during the calibration process leads to a more accurate calibration, each step of laser energy requires a manual measurement, which prolongs the length of the calibration process, and increases the risk of inaccuracies in manual measurements.
For this reason, known material calibration can be a long process, taking several hours, and requiring careful manual measurements, in order to achieve reliable results.